The sausage roll sandwich is a baked sausage roll put inside bread, and the thing that defines it is carbohydrate stacked on carbohydrate on purpose. The constant is not a fried banger this time but a sausage roll: seasoned sausage meat baked inside puff pastry until the pastry has shattered into flaking layers and gone deep gold. The variable, and the whole reason this is its own sandwich rather than just a sausage roll eaten in the hand, is the slice of soft bread it is laid into. That is a pastry already wrapped around its filling, then wrapped again in a second, softer carb. The appeal is not nutritional logic, of which there is none, but texture: a brittle, buttery, layered shell trapped against yielding white bread, the crunch held in place by the soft and made portable and tidy by it.
The build works because the two carbs are doing opposite jobs and the order keeps them honest. The sausage roll brings everything, the meat, the seasoning, the fat, and all of the texture, so the bread is deliberately plain and soft, a passive carrier chosen not to compete with the pastry but to surround and steady it. The sausage roll is usually split lengthways and laid flat rather than left as a tube, for the same structural reason a banger is split in every other sandwich in this group: a flat cut face sits stable between slices, stops the roll rolling out on the first bite, and presents the meat rather than a curve of pastry. Butter on the bread is the seal, keeping the rendered fat and pastry grease from soaking the crumb before it is eaten, and a stripe of brown or red sauce inside is the acid counter that cuts a filling which is otherwise fat-on-starch with nothing pushing back. Timing matters less than for a fried sandwich but the principle holds: a warm roll in soft bread, pressed gently, eaten before the steam has fully softened the pastry it was the whole point of keeping crisp.
The variations stay inside the pastry-in-bread frame. A cold roll versus a warm one changes the shatter; brown sauce against red is the same settled household disagreement it is on every banger sandwich; a pasty or a pie put into a soft roll is the same carb-on-carb instinct met in a different baked package. Each of those deserves its own article rather than being crowded in here.